Nuclear fuel experiment shows how liquid plutonium oxide behaves at highest temperatures

The accident at the Fukushima Daiichi nuclear power plant in Japan in 2011 led to extensive research and analysis that made nuclear energy a byword for safety. It also prompted a series of studies at the U.S. Department of Energy's (DOE) Argonne National Laboratory. Scientists want to study nuclear fuels more closely to better understand how they behave at extremely high temperatures.

In 2014, a multidisciplinary team measured and published the structure of molten uranium dioxide (UO) using the bright X-rays from beamline 6-ID-D at Argonne's Advanced Photon Source (APS), a DOE Office of Science User Facility.₂). This material is a major component of the fuel used in nuclear reactors around the world. The experiment not only provided answers but also raised questions about how liquid plutonium oxide (PuO₂) and other mixed oxide fuels considered for use in next-generation reactors would behave at similarly high temperatures.

Experimental investigation of PuO₂ raises many other safety concerns. However, the team at Argonne felt there was a fundamental need for data on actinide oxides. They took on the difficult task of designing an experiment that could overcome the complex challenges associated with studying PuO.₂.

The results of their work will help scientists and engineers model, design and build clean nuclear energy systems that continue an impressive legacy of safety.

The team of chemical engineers and physicists from Argonne, in collaboration with Stephen Wilke and Rick Weber of Materials Development, Inc. and others, published the results of their experiment “Plutonium Oxide Melt Structure and Covalency” in the April 2024 issue of Natural materials.

“Argonne is probably the only place in the world that can do this very difficult kind of experiment,” said Chris Benmore, senior physicist at Argonne. “We did the proof of concept with UO in 2014.₂and we have only now expanded the capability to include PuO₂. The experiments involve complex instruments that operate under extreme conditions.”

Benmore helped design the experimental X-ray chamber, performed the X-ray measurements, and analyzed and modeled the X-ray data. Mark Williamson, division manager of Argonne's Chemical and Fuel Cycle Technologies (CFCT) division, helped with the chamber design and safety analysis for the APS experiments and led a CFCT team that synthesized the samples for the experiments. Materials Development, Inc. designed the instrument for the measurements and made the necessary safety adjustments that made the instrument more suitable for the PuO.₂ Experiment.

Float like a butterfly, then sting like a bee

Samples of PuO₂ with a diameter of about 2 mm were floated on a gas stream and then heated with a carbon dioxide laser beam until they melted. This allowed the team to measure the structure of the samples at temperatures up to 3,000 K without risking contamination of the samples through interactions with the container. The samples initially appeared dull gray. After heating, they were shiny black. Heating the samples at different temperatures on different gas streams revealed changes in the volatility of the melt and the structure in different atmospheres.

“We deciphered the structure of liquid plutonium oxide and found that there was indeed some covalent bonding,” Benmore said. “We also discovered that the liquid structure was similar to that of cerium oxide, which can be used as a non-radioactive substitute.”

Lead author Stephen Wilke of Materials Development Inc. added: “Levitators have been used to melt materials at extremely high temperatures, but to apply this technique to studying nuclear fuel, where there are other concerns, required a much higher level of sophistication and safety scrutiny. I think it was very worthwhile.”

Another exciting result of the experiment was the use of the X-ray data to develop machine learning on a supercomputer at Argonne's Laboratory Computing Resource Center. The team was able to model what all the electrons in the system are doing with quantum mechanical precision. This work may shed further light on the nature of the binding mechanisms and help determine the safety parameters for using mixed oxide fuels in future reactors.

“The data from the combined suite of experiments not only provide information of technological significance, but also insights into the fundamental behaviour of actinide oxides at extreme temperatures,” said Williamson. “This was a fantastic collaboration of experts and an excellent example of how we work together to continuously improve nuclear energy systems.”